On the question of sterilizing (spaying/neutering) your American Bulldog

To discuss the topic of spaying/neutering, we must first preface with this:

We know of many cases of owners regretting their decision to sterilize because the dog turned out to be of exceptional quality. One example was a breeder selling a dog with conditions of sterilization. The owner complied. Then the breeder expressed regret because of the quality of the dog and asked the owner to attend shows, as this was a female and sterilization is not evident physically. This kind of situation arises because of the breeder's selfishness and is extremely counterproductive for the breed. That exceptional dog's genes should have been passed on to future generations of dogs in the breed. That is how a breed is improved.

Further, especially as a short muzzled breed, do not forget that anesthesia brings a certain health risk, including possibility of death, and that the dog must be in a proper condition before considering surgery under anesthesia. It is a surgery, and it should be treated with utmost care and seriousness.

For males, the question of castration:

If the owner does not intend to breed the male, then castration could be the right path.
Sterilization will NOT change the dog's level of obedience or socialization. It MAY affect behavior related to the male's mating behavior, including aggression toward other males, but is not guaranteed to do so.
An intact male that is not bred COULD experience stress and frustration, but that is not an absolute rule, and there are males that do not exhibit mental/emotional problems because they're intact yet not bred. This could be weighed on a case-by-case basis.

The male must have a fully developed reproductive system prior to castration. That implies somewhere between 1 year and 18 months of age.

For females, the question of spaying is a bit more complicated:

Spaying is recommended if the female is experiencing longer-than-normal heats after which she could even begin to act as if she is pregnant - producing milk, taking in toys as her puppies, etc. This is due to hormonal disbalance in the female.

Don't forget that you as the owner can adversely affect the hormonal disbalance of your dog. By giving steroid allergy shots or other hormonal treatments to mask certain problems for example. It may be quick and easy and many vets appear to love doing it, but beware.

On the other hand, if the owner can handle dealing with the twice-a-year heats then it's pretty difficult to justify spaying.

As a result of spaying the female, you introduce a brand new risk of significant weight gain and uncontrollable bladder.

Concerning spaying/neutering to reduce risk of diseases -

Most important - diseases such as cancer and problems affecting the reproductive system are highly hereditary and sterilizing a dog for the purpose of reducing risks of diseases is very questionable. Sure, you can eliminate the risk of testicular cancer by castrating the dog - you're chopping off the testicles, but you don't know what the initial genetic risk for that disease is for the given dog. Hypothetically, it would be silly if the initial risk of testicular cancer was 0.1% and you sterilized the dog to bring it down to 0.0% but you quadrupled the risk of prostate cancer as has been shown in medical studies. (Link at the bottom for summary of findings and citations)

In @Olga 's experience of breeding dozens of females over the last 16 years, in addition to scores of customers with whom she makes it a point to stay in touch, serious problems with the intact females' reproductive systems are non-existent and they tend to live to 12-13 years of age without problems - regardless of whether they were bred or not. Same goes for intact males that were never bred. Maybe some owners did not notify her of serious reproductive system issues, but that is highly unlikely, as she is her customers' go-to person for help with any problems that dogs she produced may experience. She does have lots of customers who go on to purchase their 2nd AB from her upon the natural death from old age of their first dog they purchased from her.

In conclusion:

Taking into account the pedigree of the dog and really knowing health histories of the ancestors is really most important in order to make an educated judgement regarding sterilization. That is the benefit that you got by purchasing a dog from an experienced and reputable breeder who has established his/her lines through many generations of dogs. A breeder that has purchased the female from someone else and the male from another someone else will be of no help to you, nor is that person really a breeder - that is usually an exercise in simply mixing random bloodlines with no consideration for their compatibility.

And of course in the end, the decision and responsibility of whether to sterilize lies solely with the owner, except in the rare instances in which it becomes a medical necessity as explained to the owner by a competent veterinarian. Also bear in mind that veterinary care is extremely expensive and is a very good business in the United States especially. Sterilization is good business for veterinarians.

Possibly, this is why this happened in the US:

When breeders sell their dogs with a blanket policy of not providing breeding rights to the buyer, it is unavoidable that some buyers would want to breed their dog regardless. But because of the contract, they can't go back to the breeder for any advice or consultation. Then you get generations upon generations of dogs with unknown histories that get bred left and right, causing significant deterioration of the breed. When veterinarians then see these dogs and their health issues, temperament, and whatever other genetic problems, what can they possibly advise beside sterilization? And then, sure enough, sterilization has become the norm.

The article below summarizes scientific findings with specificity and citations concerning sterilization.

Please read:
http://www.dogsnaturallymagazine.com/long-term-health-risks-benefits-spay-neuter-dogs/

Overall impression is that you introduce more health risks than the ones you eliminate as a result of sterilization. Some conclusions from the article.

"On balance, it appears that no compelling case can be made for neutering most male dogs, especially immature male dogs, in order to prevent future health problems. The number of health problems associated with neutering may exceed the associated health benefits in most cases."

"For female dogs, the situation is more complex. The number of health benefits associated with spaying may exceed the associated health problems in some (not all) cases. On balance, whether spaying improves the odds of overall good health or degrades them probably depends on the age of the female dog and the relative risk of various diseases in the different breeds."